Some of the clinical requirements of designing a radiosurgical treatment system include providing: a) a highly precise beam delivery to targets located throughout the body, b) a highly conformal dose distribution, c) the ability to irradiate both small and/or large complex-shaped lesions. To do this, the system uses a combination of beam positions whose relative beam weights, or dose contributions, have been scaled to volumetrically shape the dose accordingly. One or more collimators can be selected to give the beams the required shape to achieve the desired clinical result. Collimators are used in linear accelerators (LINACs) of radiation treatment systems, such as radiosurgery or radiotherapy systems. The collimators help to shape the beam of radiation emerging from the LINAC, and they can limit the maximum field size of the beam. The treatment head of the LINAC typically includes both a primary collimator and a secondary collimator.
Current radiation treatment surgery system such as the CYBERKNIFE® radiosurgery system, manufactured by Accuray™ Incorporated, USA, relies on a set of twelve circular, fixed-aperture secondary collimators to reduce the beam to the size required by the treatment planning algorithm. Because the apertures of these secondary collimators are of fixed-size, one fixed-aperture collimator individually does not provide flexibility of having an aperture of different sizes. The set of fixed-aperture secondary collimators are used to provide this flexibility; however, when a different size of aperture is to be used in the radiation treatment system, the fixed-aperture collimator needs to be changed. Currently changing of these collimators is accomplished manually by hand.
FIGS. 1 and 2 illustrate the conventional process of manually changing fixed-aperture collimators. FIG. 1 shows a locking nut 101 being removed by unscrewing the locking nut 101 and releasing the fixed-aperture collimator 102 from a collimator housing 103. Subsequently, the fixed-aperture collimator 102 may be replaced by another collimator by screwing the locking nut 101 to the collimator housing 103. In this implementation of changing fixed-aperture collimators with CYBERKNIFE® radiosurgery system, manufactured by Accuray™ Incorporated, USA, the locking nut 101 has to be rotated by hand to release the fixed-aperture collimator from off the end of a LINAC. FIG. 2 illustrates the fixed-aperture collimator 102 released from the collimator housing 103 during a manual process of changing the fixed-aperture collimator.
One conventional process for manually changing the collimators includes the following operations. First, the operator removes the locking nut 101 by unscrewing it from the collimator housing 103. Next, the operator, using his/her hand, supports the fixed-aperture collimator 102 from the bottom. Next, the operator releases a retaining pin to release the fixed-aperture collimator 102 from the collimator housing 103 to the operator's hand. Next, the operator selects a different fixed-aperture collimator and inserts the different fixed-aperture collimator into the collimator housing 103 until the retaining pin clicks. Lastly, the operator replaces the locking nut 101 and screws the locking nut 101 to the collimator housing 103. In this conventional process, the locking not 101 has to be turned multiple turns to be removed from the end of the LINAC.
Another conventional type of collimator, illustrated in FIG. 3, is an IRIS collimator 300, which includes a mechanism for varying the IRIS aperture 301 of the collimator 300 to adjust the beam width in radiosurgery and radiation therapy applications. The IRIS collimator 300 is typically used as secondary collimator. The IRIS collimator 300 includes multiple leaves 302 that are each driven linearly, which collectively closes and opens the IRIS aperture 301 of the collimator 300, varying the size of the IRIS aperture 301. Although, the IRIS collimator 300 is configured to provide flexibility in varying the size of the aperture 301 of the collimator, however, practical implementations of the IRIS collimator 300 suffer from having an aperture that is non-circular due to the mechanical limitations in the number of moving parts of the IRIS collimator 300.